Document EP-A-0324430 discloses a composition that is extrudable and curable, comprising a continuous matrix of a thermoplastic polymer material containing hydrolysable silane groups and fine particles of a vulcanized (or cured, or cross-linked) rubber which are dispersed in said continuous matrix.
The extruded composition is cured in the presence of water. It then presents excellent mechanical properties and is very temperature stable because of its curing. It can be used particularly, but not exclusively, for making the sheaths of electric cables and wires.
According to that document, the thermoplastic polymer material containing the hydrolysable silane groups is preferably initially grafted by an unsaturated and hydrolysable silane compound. The thermoplastic polymer material used for silane grafting is, in particular, a polyethylene or a copolymer of ethylene and at least one .alpha. olefin containing 2 to 6 carbon atoms, or a copolymer of ethylene and an unsaturated ester of a carboxylic acid. The unsaturated and hydrolysable silane compound used may be, for example, vinyl trimethoxy silane, vinyl triethoxy silane, or a vinyl tri(methoxy and ethoxy) silane. It is grafted on the thermoplastic polymer material in the presence of a peroxide.
The material is cured by hydrolysis and condensation of the alkoxy groups. In order for such curing to take place, the sample is immersed in water at 90.degree. C. containing a catalyst for condensing the silane groups. In another variant, the sample is stocked in moist air for several weeks to cure it. Under such conditions, curing is caused by a catalyst for condensation of silane groups, added in the form of a master mixture to the base composition.
In a variant, the thermoplastic polymer material containing the hydrolysable silane groups is a copolymer of at least one a monolefin preferably containing 6 to 8 carbon atoms, and a silane that is unsaturated and hydrolysable. A preferred such copolymer is an ethylene-vinyl silane copolymer.
In the majority of examples given in that document, the composition is fabricated in two steps: the first for grafting silane on the thermoplastic polymer material, and the second for dynamic curing of the rubber and for dispersion thereof in fine particles within the continuous matrix of the thermoplastic polymer material containing the hydrolysable silane groups.
In one of the examples given in that document, the composition is fabricated in a single step, by placing together in a mixer, a mixture of thermoplastic polymer material, hydrolysable unsaturated silane, and peroxide, in particular high density polyethylene, vinyl triethoxy silane, and peroxide, and a mixture of natural rubber with a vulcanizing agent, and processing them together at 180.degree. C. for several minutes. Nevertheless, fabricating the composition in a single step in accordance with that example gives rise to the problem of reactional interference between firstly the thermoplastic polymer material and the elastomer and secondly the other ingredients of the two mixtures. A priori, the properties of the composition fabricated in that way are not analogous to those of compositions fabricated by starting from the thermoplastic polymer material initially grafted with silane, but are substantially degraded relative thereto.
In document EP-A-0 181 735, the curable composition is composed of a nitrile rubber and a thermoplastic including hydrolysable silane groups. That composition is cured, as before, in the presence of water and of a catalyst for condensing the hydrolysable groups. In a variant, the thermoplastic contains polymerized units, and a hydrolysable silane compound is added to the mixture to react with the thermoplastic and to include hydrolysable silane groups in the thermoplastic. Curing takes place as before and is very lengthy.